A student is investigating the phenomenon of sound reflection in air at normal room conditions.
(i) Define the term “echo” as understood in acoustics.
(ii) State the time–delay condition that must be satisfied for a listener to perceive a distinct echo of a sound produced in air.
(iii) Using reasonable everyday estimates for the speed of sound in air and the minimum perceptible time gap for the human ear, give a reasoned argument to show that an obstacle must be at least of the order of a few tens of metres away from the source for an echo to be heard distinctly.